The present invention relates to a method and an apparatus for testing a photomask which is used in fabricating a semiconductor device, by an electron beam probe.
A mask pattern on a photomask used for fabricating a large scale integration (hereinafter referred to as an LSI) becomes fine as the packing density of the LSI is made higher. Accordingly, a high dimensional accuracy and a high positional accuracy are required for the mask pattern.
In order to test such a fine mask pattern on the photomask with high accuracy, there have been proposed various electron beam testing apparatuses. As one of the above testing apparatuses, for example, a scanning electron microscope (hereinafter referred to as an SEM) is well known which is described on pages 37 to 45 of the SPIE, Vol. 100, Semiconductor Microlithography II, 1977.
In the above SEM, recoil electrons and secondary electrons, both of which result from the physical interaction between an electron beam probe and a photomask, are detected by a collector to generate signals having a waveform corresponding to the surface state of the photomask. The boundary between a mask pattern and the pattern-lacking region of the photomask is detected from the above signal waveform, and such positional information of the mask pattern as the width of pattern and the position of pattern is obtained by using the above boundary. It can be judged from the comparison between the positional information of mask pattern and design data of the photomask whether the mask pattern is fabricated in a satisfactory manner or not.
Now, the conventional method of detecting the width or position of a mask pattern on the basis of the signal waveform made by recoil or secondary electrons will be explained below in more detail by reference to FIGS. 2a and 2b of the accompanying drawings. In FIG. 2a, reference numeral 19 designates an opaque pattern which is formed on a glass substrate 18 through evaporation and is made of a metal such as chromium. FIG. 2b shows a signal waveform 60 delivered from an electron collector when the mask pattern 19 is scanned in the x-direction over a length l' with the center at a specified position x.sub.p. The signal waveform 60 is compared with a threshold level 61 for judging the boundary between the mask pattern and the pattern-lacking region of the photomask. When the intersecting points of the signal waveform 60 and the threshold level 61 are given by x.sub.1 and x.sub.2, respectively, the width l of the mask pattern 19 can be expressed by the following equation: EQU l=x.sub.2 -x.sub.1.
Further, when the specified position x.sub.p is located at the center of the pattern, the X-coordinate x.sub.p of the specified position satisfies the following equation: EQU x.sub.p =(x.sub.1+ x.sub.2)/2.
In the conventional testing method which has been explained in conjunction with FIGS. 2a and 2b, the photomask is scanned by the electron beam probe over a length l' which is longer than a width l of a mask pattern 19 to be tested. Accordingly, the transparent region of the photomask having no mask pattern is also irradiated with the electron beam, and floating hydrocarbons in the SEM adhere to that part of the transparent region of the mask which is subjected to the irradiation of electron beam. In other words, the irradiation of electron beam gives rise to contamination on the transparent region of the photomask. The number of contaminated points is increased as the scanning motion of the electron beam is carried out at a larger number of parts of the photomask for the purpose of enhancing the accuracy of test, and undesired, opaque patterns are formed on the photomask when the number of contaminated points is increased to a certain degree.